In general, liquid developers for electrophotography are obtained by dispersing organic or inorganic pigments or dyes, such as carbon black, nigrosine or phthalocyanine blue, and a natural or synthetic resin, such as an alkyd resin, acrylic resin, rosin or synthetic rubber, in a liquid which has good insulating properties and a low dielectric constant, such as a petroleum based aliphatic hydrocarbon, and adding a polarity suppressing agent such as a metal soap, lecithine linseed oil, higher fatty acid or polymer which contains vinylpyrrolidone.
In developers of this type, the resin is dispersed in the form of insoluble latex particles with a particle diameter from a few nms to a few hundred nms, but in a conventional liquid developer there is inadequate bonding between a soluble resin which is used for dispersion stabilization purposes or the polarity controlling agent and the insoluble latex particles, as a result, soluble resin for dispersion stabilization purposes or the polarity controlling agent readily diffuses into the solution. Consequently, the soluble resin for dispersion stabilization purposes becomes separated from the insoluble latex particles on long term storage or repeated use and the particles may sediment, coagulate or lump together and the polarity becomes indistinct. Furthermore, it is difficult to redisperse particles once they have been sedimented or formed into lumps, as a result, the particles tend to stick to certain parts of the developing apparatus and they may contaminate the image parts or causes a breakdown of the developing machine by blocking pumps, for example.
It has been suggested that the insoluble latex particles should be chemically bound to the soluble resin for dispersion stabilization purposes an attempt to eliminate these disadvantages, and disclosures to this effect have been made, for example, in U.S. Pat. No. 3,990,980. However, although such liquid developers are somewhat better in terms of their dispersion stability with respect to the sedimentation of the particles themselves, the effect is not sufficient, and when these developers are used in actual developing apparatus, the toner becomes attached to various parts of the apparatus and solidifies in a film like coating and, moreover, there is a problem in that redispersion is difficult. This can lead to breakdown of the apparatus and contamination of the transferred images. Furthermore, the combinations of dispersion stabilizers and insolubilized monomers which can be used to prepare mono-disperse particles with a narrow particle size distribution is very limited in the methods of manufacture of resin particles disclosed in the above mentioned documents. Particles which have a wide particle size distribution containing large numbers of large, coarse particles or polydisperse particles in which two or more average particle sizes are present. Furthermore, it is difficult to obtain the prescribed average particle size with particles in a mono-dispersion which has a narrow particle size distribution, and large particles of at least 1 .mu.m, or very fine particles of less than 0.1 .mu.m, are formed. Moreover, there is a further problem in that the dispersion stabilizers which are used must be prepared using a complicated and time consuming process.
Additionally, methods of overcoming the above mentioned problems in which the degree of dispersion of the particles, the .redispersion properties and the storage properties are improved by using insoluble dispersed resin particles of copolymers of insolubilized monomers and monomers which contain long chain alkyl groups or monomers which contain two or more polar components are disclosed, for example, in JP-A-60-179751 and JP-A-62-151868. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".) Furthermore, methods in which the degree of dispersion of the particles, redispersion properties and storage stability are improved by means of insoluble dispersed resin particles comprised of copolymers of insolubilized monomers and monomers which contain long chain alkyl groups in the presence of polymers in which difunctional monomers have been .used or monomers in which macromolecular reactions are used have been disclosed, for example, in JP-A-166362 and JP-A-63-66567.
On the other hand, techniques in which more than copies are printed using offset printing master plates obtained using electrophotographic techniques have been introduced in recent years, and progress has been made in improving the master plates in particular and it is now possible to print in excess of 10,000 copies even with large plate sizes. Furthermore, progress has been made with shortening the operating time of the electrophotographic plate making system and improvements have been realized with the speeding up of the development/fixing processes.
The dispersed resin particles manufactured using the procedures disclosed in the aforementioned JP-A-60-179751, JP-A-62-151868, JP-A-62-166362 and JP-A-63-66567 do not always provide satisfactory performance in respect of particle dispersion properties and redispersion properties when development speeds are increased, and in respect of printing resistance when the fixing time is shortened or when the master plate is large (for example A3 size or greater).
The problems of conventional liquid developers of the type described above are solved by the present invention.